Transcript
Introduction to Distributed Generation Power Quality Standards
Good afternoon, everyone, and welcome to today’s white paper webinar. Today we’re gonna be talking about power quality standards as it relates to distributed generation. This paper describes a little bit of the history of these standards and which one to choose now, because there are several to choose from at this point, unlike the case just a few years ago.
These are rapidly changing standards. The two newest ones are still undergoing new drafts. So this is a moving target, just like distributed generation itself is a moving target for utilities.
IEEE 519 and Early Concerns About Harmonics
Early on, being even just 10 years ago, one of the biggest concerns about inverter-based resources, in particular solar and wind turbine systems that feed into a DC bus and then use inverters to generate synthetic 60 Hz, was waveform distortion, in particular harmonics. So the first standard that was popular that talked about distributed generation was IEEE 519, the standard that sets limits for voltage and current harmonics and waveform distortion. One of the biggest concerns was the amount of distortion these inverters would cause because of their nonlinear nature.
When the 2014 edition of 519 came out, it included a note for the harmonic limits when there’s distributed generation. Others shown here in figure one, as seen on our power quality ruler. That’s a six-inch ruler that’s filled front and back with PQ information. Here are the limits for voltage and current as we see in figure one.
For voltage, there are different levels for different bus voltages, and it’s the same percentage as a fundamental for individual harmonics and then for THD, total harmonic distortion. However, for current, it’s more complicated. There are different sets of limits for different ranges of harmonics, second through the 11th, 11 through 17th. There are different values for some of the lower order, even harmonics.
The most important issue here is that there are different thresholds for different ratios. This is the ratio of source circuit current to max demand load current. The standard talks about how do you compute the max demand load current. But this is a measure of how stiff the system is and how much one customer’s distortion could affect another customer.
Footnote C and Distributed Generation
The important thing for distributed generation, it was footnote C that states that if the customer has any sort of local generation, then they have to use the most strict standards, and that’s top row here, the under 20 row. So if per 519 2014, they have to use the most strict row, the under 20. These are very tight limits, just a few percent for individual harmonics and 5% for total demand distortion.
That was put out in 2014 with that footnote C, and at the time, that was really the only PQ standard that talked about distributed generation and power quality.
Newer Standards: IEEE 1547 and IEEE 2800
Since that time, we have two newer standards. We have IEEE 1547 and IEEE 2800. Those have been published since the 2014 edition of the 519 standard. Consequently, the very latest edition of 519, which came out just about a year ago in 2022, clarifies when to use 519 or when to use the other standards.
Flowchart for Choosing the Right Standard
I’m gonna jump to a separate presentation that goes into a little more detail with the same topics here as in this paper. This is a little bit about each standard, and I’m gonna jump to the flowchart that’s given in the 519 standard. This is to guide you to which standard to use if you have distributed generation when it comes to power quality.
This is in the 519 2022 standard because the concept is the 519 standard is meant to be a load standard, not a generation standard. So the flowchart says use 519 if the rated generation is less than 10% of the average annual load demand. What they mean is if it’s mostly load and not mostly generation, use 519, because 519 is a load standard, not a generation standard.
This is a pretty nuanced statement here. This is the rated generation, not the amount of generation they actually produce, but what they could produce. And then this is less than 10% of the annual average load demand. So if that condition is true, then go ahead and use the 519 standard because the site is mostly load and the generation isn’t a significant fraction of that type of system.
Otherwise, use IEEE 2800 if it’s transmission connected generation, or use 1547 for distribution connected generation.
Inverter-Based vs. Rotating Generation
There’s one other twist. The 2800 standard only applies to inverter-based resources, IBRs, a generation that uses inverters to produce synthetic 60 Hz. If it’s rotating generation, then 2800 doesn’t apply. There are other standards for transmission connected rotating generation. That’s a well-developed field. But 1547 applies to both rotating and inverter-based generation.
So on the distribution side, use 1547 regardless of the type of generation, but on the transmission side, you’d only use 2800 if it’s inverter-based generation, IBR, inverter-based resource.
Battery Energy Storage Systems
One thing that isn’t addressed in this chart is what if it’s kind of a hybrid? What if it’s a battery storage system? For example, a system that charges the batteries at night when the power cost is low and supplies power during the day during times of peak demand. So that means at night it’s a load, during the day it’s generation. Right now that kind of falls through the cracks. It’s an open question as to which standard or standards you might apply to that. So that sort of battery energy storage system is not really well described by any of these standards at this point.
Scope of Each Standard
519 is meant to provide voltage and current limits to both utilities and to loads their customers have. The intent behind 519 is to limit one customer’s ability to cause voltage distortion for another customer. And for that reason, the measurement point is the point of common coupling where your customers are connected together. The other two standards have a different focus.
1547 is the requirements for distributed energy resource inverters to connect to distribution. This talks about a lot more than power quality. Power quality is one clause in this document. There’s a whole lot of other aspects to this document that include disconnect requirements under certain conditions, islanding detection, test and verification requirements, requirements for communications and for different control mechanisms that are in the standard. So 1547 covers a lot more than power quality requirements, but it is one aspect, is the power quality in particular.
Another useful document is 1547.2. This is an application guide that gives you some context on why 1547 is the way it is. It gives some rules of thumb, some tips and tricks. It has some background information. This is a little bit dated now because 1547.2 came out in conjunction with the first version of 1547, the 2003 edition. And there’s been a whole lot of changes in 1547 between 2003 and 2018. So some of the information in the application guide is a little bit dated, but still worth reading and worth having as a reference.
2800 is similar in many ways to 1547. One of the key ones again is it only applies to inverter-based generation, not rotating generation. And it also gives requirements that go beyond just power quality: protection requirements, performance requirements, monitoring requirements, communications requirements that inverters have to meet in addition to the power quality.
Power Quality Requirements Beyond Harmonics
Back to the white paper we discussed is some of the other aspects of power quality that these require. For example, in 1547, there are no limits for voltage distortion on either one of these two standards, unlike 519. But the power quality requirements in 2800 include DC injection limit, limits on flicker, rapid voltage changes, overvoltage and peak voltage.
1547 is similar in the scope of what it covers. It covers limits on absolute peak voltage, cumulative time spent above a peak voltage level, limits on rapid voltage changes, which happen when the inverter either switches on or off or makes step changes in its output level, and also limits on flicker contribution, amount of flicker it emits in terms of PST and PLT, which of course references the 1453 standard for flicker. These requirements are similar in 1547 but not exactly the same. Limits in 2800 are in general a little bit tighter, given that it’s a transmission.
Comparing the Three Standards
Here is a table comparing the scope of power quality issues from these three standards. We have 519, we have 1547, and we have 2800. Use 519 again if it’s mostly load and just a little bit of generation. Otherwise, use 1547 if it’s connected to distribution, or use 2800 if it’s transmission connected.
Only 519 includes voltage harmonics limits. The others are just limits on current. Some key differences in the harmonic measurement techniques on these three standards: 519 is based on the ratio of short circuit current and max demand load current. That’s a measure of how stiff your system is compared to the customer’s load.
That is quite a bit different when we get to these other two standards, 1547 and 2800. One, they include interharmonics, whereas 519 only includes harmonics. So it doesn’t include non-synchronous components, but these other two do. And instead of total demand distortion like we have in 519, we have total rated distortion, something that includes interharmonics, but it also is sized based on the maximum generation it can provide. Not the generation it actually does provide, but its rated generation.
Even with the same measurement techniques, you’ll get different numbers because you’re comparing TRD with these two standards and TDD, total demand distortion, with 519. You have slightly different measurements with these different standards. 519 doesn’t include interharmonics. These other two do, and those interharmonics relate to non-synchronous energy that inverters might produce.
Of course, 519 only covers distortion, so you’re not going to see limits on flicker, rapid voltage change, overvoltage, or peak voltage limits in 519. That’s out of scope for 519. But these standards aren’t limiting themselves to waveform distortion. These are all about distributed generation. So these do have limits on the amount of flicker contribution from the inverters, the amount of rapid voltage change these produce, and the amount of overvoltage, both from an instantaneous standpoint on a waveform and from RMS voltage changes. Those are all limits that are applied at the point of connection for 1547 and 2800.
So these are definitely not interchangeable, especially there is a big difference between 519 and the other two, in that the other two include many other quality metrics. But even within these two, the limits are a little bit different from each other. They’re not interchangeable.
Inverter Compliance Modes
Inverters can be programmed to meet the requirements of one or the other. Some of these inverters have adjustments that can make them 1547 compliant or 2800 compliant. And some of those changes don’t really relate to power quality issues, but rather to disconnect requirements. They must disconnect within so many milliseconds of certain conditions happening. Those conditions and requirements are a bit different in the two standards, and so some inverters can be put into different compliance modes based on which standard applies because the inverters may be used at different voltage levels.
Including Standards in Tariff Agreements
One of the key aspects of using these is to make sure you adopt these and they have some teeth. One of the purposes of these standards is to make sure if you’re a utility that your customers that are connecting distributed generation aren’t causing problems on the grid. The purpose of these standards is to make sure that these inverters don’t cause problems on the grid.
If they are causing problems, you have to have some teeth in terms of the enforcement of these standards. If a generation site is exceeding the quality standards here, you have to be able to push back on them and basically get them to spend money to fix that problem and to mitigate that. The way to do that is to include these standards in your tariff agreement so they agree to these at the time they sign up for these programs.
Some utilities are in the process of doing that now, and if you are, you want to do this in a deliberate fashion. I’ve seen several different ways utilities do this. Some specify a standard but don’t specify a particular edition. Like, they may write in you must abide by the requirements of IEEE 519, but they don’t specify which edition. It could mean the 2014 edition, the 2022 edition. You don’t want to leave that to chance.
Another approach is to specify a certain edition, like specify 519-2022, the very latest edition. Another approach is to leave it kind of open, as in, like, the most recent edition at the time of connection. That way you’re not locked into an old standard if a newer one comes out, which these are constantly changing. Every few years there’s a new edition.
So you might choose option two or option three, and there are pros and cons to both. With option two, you know what you’re getting. That’s not a moving target. If you just have it as the most recent edition, there may be changes that are difficult for the utility that were unexpected, and then you’re kind of stuck with those, so you don’t know what you’re agreeing to if it’s the most recent edition. Or it may be customers are using one edition, some customers are using a different edition if it changes, and so now you’ve got a mix of standards out there. But there are pros and cons to both approaches. What you don’t want to do is have it be random. You want to intentionally decide which approach you use in terms of writing this into your tariff agreements. There’s not necessarily a right or wrong answer, but you want to make sure you have a good reason for picking one or the other.
Summary and Harmonic Limits Comparison
To wrap it up, there are really three standards that exist for power quality for distributed generation: 519, 1547, and 2800. Originally, 519 was the only one, but with publication of 2800 and 1547, now we have several to choose from. The way to do this is to use the flowchart that appears in 519. And again, 519’s a load standard. If it’s mostly load, use 519. Otherwise, 2800 for transmission connected generation or 1547 for distribution connected.
Here’s a little bit of a summary of the different harmonic limits that we see in 2800, 1547, and 519. You can see in some cases the limits are a little bit the same, but we have limits on TRD, not TDD. And if you consider this top row is the most strict row of the 519 standard, that’s where these are the closest, where we’re talking 4% and then 2%. That’s very similar to the 519 table.
So it may look like these are the same limits, but the even harmonics are not quite the same, and these are all as a percent of total rated distortion. These all include interharmonics, unlike 519 that only includes synchronous distortion harmonics. And you can see that with 2800 there are different levels for different bus voltage, and they start off somewhat similar to 519 but start to deviate as you get into higher bus voltages. But again, 2800 standard is also based on total rated distortion, just like 1547, not total demand distortion.
There’s also some interesting fine print here in 2800 where the current limits only apply if the voltage imbalance is within a certain amount. If the negative sequence component of voltage is less than 2%, then these limits apply. If the voltage imbalance is over 2% and it’s measured by symmetrical components, then all bets are off. So that’s an interesting twist there that you have to have your voltage imbalance reasonably low for these limits to apply.
Conclusion
That wraps up the topic of the paper. Really the bottom line is you want to be using the correct standard for distributed generation. There are three different standards for these types of systems, and most important, you want to have these as part of your tariff agreements with customers so that there’s some teeth to these standards. You’re not just knowing that a customer’s creating problems for others. You have an ability to push back and have that problem be mitigated.
If you have any questions about the white paper, give us a call anytime at 1-800-296-4120 or send an email to support@powermonitors.com. Also, if you’d like a power quality ruler with the harmonic limits on them plus flicker and all sorts of other PQ information, send an email to support@powermonitors.com. Thanks for attending, everyone. Everyone have a great afternoon.